This invention relates to wire printing apparatus. Such apparatus prints by projecting a printing wire towards a record medium and making an impression in the form of a dot on the record medium. Usually the dots are arranged as characters of the well-known dot-matrix type.
In the usual arrangement the wire is projected towards the record medium by an actuating mechanism consisting essentially of an electromagnet and an armature. The armature is normally held in an open position with the back end of the printing wire biassed against it. When it is desired to cause the print wire to make a dot, current is passed through the coil of the electromagnet and the armature is attracted towards the electromagnet, carrying the print wire with it. The armature is brought to a stop against the electromagnet but the print wire continues, projected towards the record medium with the kinetic energy already imparted to it.
The source of the current passed through the coil of the electromagnet is a drive circuit. Hitherto, drive circuits have used predetermined voltage drive circuits and predetermined current drive circuits. In a voltage drive circuit the coil is switched between two voltage rails for a predetermined time. During this period the current rises at a rate determined by the voltage and the inductance and resistance of the coil. In a current drive circuit the arrangement is similar, but the peak current is limited to a predetermined value.
These circuits are relatively inefficient, defining efficiency for this purpose as the percentage of the energy supplied to the drive circuit that is passed on to the print wire. The more inefficient the drive circuit, the larger is the power supply that is necessary and the greater is the heat dissipation within the head assembly. This last factor is especially disadvantageous as limiting the repetition rate at which drive signals can be applied to the head.